1. Field of Invention
The present invention relates to an ohmic contact electrode for an n-type semiconductor cubic boron nitride.
2. Description of Related Art
Attention has been given to a semiconductor cubic boron nitride as a new material for semiconductor devices such as diodes, transistors, sensors, and so on. Although an insulating cubic boron nitride is known broadly, that which is discussed here is a semiconductor cubic boron nitride having low resistivity.
A cubic boron nitride has a wide forbidden band width (7.0 eV) and a high heat-proof temperature (1300.degree. C.), and is chemically stable. Consequently, a semiconductor cubic boron nitride has been strongly expected to be an excellent material for environment-proof power devices, and blue light emission elements.
A cubic boron nitride does not naturally exist but can be formed through high-pressure synthesis. Further, recently, the formation of thin film through vapor phase synthesis has been reported.
A p-type cubic boron nitride can be obtained by doping beryllium (Be).
An n-type cubic boron nitride, on the other hand, can be obtained by doping sulfur (S) or silicon (Si). At present, a pn-junction diode is produced for trial by using a high-pressure synthetic method in such a manner that an n-type cubic boron nitride is continuously grown with a p-type cubic boron nitride as a seed crystal. The pn-junction diode which is produced has a diode characteristic under 650.degree. C. Further, it has been reported that a pn-junction diode emits, by injection of minority carries, light from ultraviolet to blue centering around 340 nm.
A contact between a metal and a semiconductor is either a Schottky contact or an ohmic contact. The Schottky contact has a rectification property so that a current does not flow in a reverse direction. In the case of producing semiconductor devices, it is very important to form electrodes which can be formed by ohmic contact. The term "ohmic contact electrode" means an electrode in which the characteristic of a current flowing through the electrode and the characteristic of a voltage across the electrode are symmetrical forward and backward in accordance with the Ohm's law. Further, it is preferable that the contact resistance is reduced as much as possible. The term "contact resistance" is defined as a voltage to be applied so as to make a unit current flow through a unit contact surface. The unit of the contact resistance is .OMEGA..cm.sup.2.
There has not yet been found a superior electrode for an n-type cubic boron nitride. Although silver (Ag), silver paste, or the like is used as an electrode for an n-type cubic boron nitride ((1) R.H.Wentorf, Jr.: J. of Chem. Phys. Vol. 36 (1962) 1990; (2) O. Mishima, etc.: Science Vol. 238 (1987) 181), there has been no report that an ohmic contact could be obtained.
The technique for forming ohmic contact is necessary and indispensable to produce semiconductor devices. Further, if an ohmic contact is obtained, it is desirable that the contact resistance in the ohmic contact portion be reduced as much as possible. The contact resistance of general electron devices is 10.sup.-2 .OMEGA..cm.sup.2 or less, and a smaller contact resistance of 10.sup.-4 .OMEGA..cm.sup.2 or less is required in high-speed and high-frequency devices.
If a formed contact is not an ohmic one but a Schottky one, carriers cannot be effectively injected into a produced device because of the existence of a Schottky barrier in the contact electrode portion even when the current is to be made to flow in the device. This results in low efficiency of carrier injection. Further, the voltage drop across the contact electrode portion is remarkably large because the resistance is larger at the contact electrode portion. Therefore, the effective voltage applied to the device ends up being small. As a result, the particularly desireable characteristics discussed above for the device cannot be obtained. Further, the generation of heat in the contact electrode portion is a large problem.
Accordingly, the formation of an ohmic contact electrode is necessary and indispensable in order to make it possible to utilize a semiconductor cubic boron nitride as a material for semiconductor devices.
An object of the present invention is therefore to form an ohmic contact electrode on an n-type semiconductor cubic boron nitride.